쉘 스크립트 연산식 질문드립니다 (내용 간단해요!)

day=0 mon=0 year=0
reamin=0
a=("일" "월" "화" "수" "목" "금" "토" )
month=(31 28 31 30 31 30 31 31 30 31 30 31)
echo -n "year : "
read year
echo -n "mon  : " 
read mon
 
allday=$[ (year-1)+(year-1)/4-(year-1)/100+(year-1)/400 ]
i=0
for ((i=0; i < $mon; i++)); do
if (( $i == 1 && `expr ($year % 4)` == 0 && `expr ($year % 100)` != 0 || `expr $year % 400` == 0 ))
    then
        $month[1]=`expr $month  + 1`
    fi
    $allday=`expr $allday + 1`
done
$allday=`expr $allday - $month[$i]`
remain=`expr ($allday + 1) % 7`
$i=`expr ($remain + day - 1) % 7`
 
c=0
d=0
count=1
 
echo -n "  일  월  화  수  목  금  토"
for ((c=0; c<6; c++)); do
    for ((d=0; d<7; d++)); do
            if [ $i -eq 0 -a $d -lt $reamin ]
            then
                echo "    "
                continue
            elif [ `expr $count -ge $month[mon-1] + 1` ]
                break
            fi
            echo "    "
            echo $count 
    done
    echo -n ""
done